also include aquifers or parts of aquifers in 

 geologic formations extending some distance sea- 

 ward from the coasts, but which may be cut off 

 from land recharge by clay beds or faults. 



—There is some suggestion that fresh water in 

 deposits offshore may originate from processes 

 involved in compaction of sediments and where 

 the water is genetically unrelated to the land 

 hydrologic system. This may be considered as a 

 source of "new" water. 



Extensive aquifers of the first type occur along 

 the middle and south Atlantic coasts and the Gulf 

 Coast; elsewhere there are few possible aquifers 

 worth exploring. The reason is that the rocks lack 

 permeability or that the outcrops are too narrow 

 and extend relatively short distances offshore. All 

 these aquifers contain fresh water in their land- 

 ward parts and salty water in their seaward parts. 

 The boundary between the two waters can be 

 either onshore or offshore. The parts of these 

 aquifers that contain fresh water do not constitute 

 a "new" source of water. This does not mean that 

 the aquifers might not be utilized more efficiently 

 and more water taken from them. Typically, plans 

 for more efficient management would be based on 

 a determination as to the maximum rate which 

 fresh water could be pumped without inducing 

 encroachment of salt water; or, where encroach- 

 ment could not be prevented at the desired 

 withdrawal rate, prediction of the rate at which 

 salinity would increase so that appropriate treat- 

 ment and processing techniques could be sched- 

 uled. 



C. Desalted Water 



Sea water or other saline waters may be 

 converted to fresh water by one of several pro- 

 cesses that either remove the sweet water from the 

 salt solution, or remove the salts from the solu- 

 tion. In either case, a brine that is more concen- 

 trated than the salt solution fed into the conver- 

 sion plant is a coproduct with the fresh water and 

 must be disposed of. 



Some methods of producing fresh water from 

 salt water have been known for centuries; other 

 methods have been developed only very recently. 

 The costs and rate of conversion of water vary 

 widely among the processes. Of the processes that 

 are now actively being studied and developed in 



the United States some are now utilized commer- 

 cially, some are in the pilot plant stage, and others 

 are being studied in laboratories. 



Conversional processes may be divided into 

 four broad categories:'' 



-Distillation or evaporation processes, which in- 

 volve the conversion of salt water into sweet 

 water vapor and then its condensation. The major 

 process types in this group being investigated 

 under the Office of Saline Water (OSW) program 

 are: 



a) multiple-stage flash (MSF) distillation; 



b) vertical tube evaporation (VTE); 



c) vapor compression; 



d) liquid-liquid heat transfer; 



e) multi-process systems. 



—Membrane processes, which employ methods by 

 which diffusion proceeds through use of a 

 semipermeable membrane. Salt and water are 

 separated while still in a liquid state. The major 

 process types being investigated under the OSW 

 program are: 



a) electrodialysis (ions pass through); 



b) reverse osmosis (water passes through); 



c) transport depletion; 



d) electrosorption and desorption; 



e) forced flow transport depletion. 



—Crystallization processes, which involve the sep- 

 aration of pure water solids (ice) from a salt 

 solution. The major types being investigated under 

 the OSW program are: 



a) vacuum freezing vapor compression; 



b) order of melting inversion. 



—Other processes. Within the last 10 years OSW 

 has investigated some 47 processes, of which all 

 but 15 have been eliminated because they were 

 not economically competitive with other desalting 

 systems. In addition to those mentioned previ- 

 ously, studies are continuing on conventional ion 

 exchange, conventional reverse osmosis, thin chan- 

 nel reverse osmosis, and pressure dialysis. 



Expenditures of OSW on desalting research and 

 development from FY 1953 through FY 1968 

 amounted to about $118 million, of which about 



Summarized from information supplied by the Office 

 of Saline Water, Department of the Interior, 1968. 



VII-225 



